The present invention relates to an exhaust gas purifying system of an internal combustion engine, and more particularly to a technique for preventing a particulate filter and a NOx trap catalyst from receiving excessive heat load during the recovery processing of these filter and catalyst.
Particulate filters and NOx trap catalysts are common known as traps for removing specific contents from exhaust gas of an internal combustion engine. Each particulate filter has built in a filter element produced by molding ceramic into a honeycomb monolith. The filter element filters out particulates from exhaust gas. Each NOx trap catalyst changes its property according to the air/fuel ratio such as to remove NOx in exhaust gas by trapping NOx in the catalyst when the air/fuel ratio is lean. Such a NOx trap catalyst also traps sulfur content in exhaust gas in addition to NOx. These particulate filter and NOx trap catalyst are required to execute a recovery processing for recovering their performances when the accumulated quantity of eliminated objects such as particulates reaches a predetermine. If the engine is operated without executing the recovery processing of these filter and catalyst, there will cause an undesired increase of an engine back pressure and an undesired discharge of exhaust gas including NOx into atmosphere. Further, the NOx trap catalyst is required to execute recovery processing (desulfurization recovery processing) for desulfurizing sulfur content trapped by NOx trap catalyst in addition to NOx.
Japanese Published Patent Application No. 2002-155793 discloses typical recovery processing of a particulate filter and a NOx trap catalyst wherein particulates trapped by the particulate filter are burnt by raising an exhaust gas temperature at a higher temperature than that during a normal operation, and NOx and sulfur content trapped by the NOx trap catalyst are discharged by temporally changing the air/fuel ratio.
Japanese Published Patent Application No. 2000-179326 discloses a method of increasing an exhaust gas temperature by retarding a main injection timing, by executing a post injection, and by increasing a quantity of exhaust gas recirculation, for the recovery processing of a particulate filter and a NOx trap catalyst.